This invention relates to an image forming apparatus including means for dehumidifying and, more particularly, to an arrangement capable of high-quality image formation without being influenced by environmental humidity in an image forming apparatus, which has a recording sheet supply section and an image forming section, and which forms an image on a recording sheet of paper.
Various types of image recording apparatuses which form an image on various paper recording sheets are well known, and are used in practical applications. However, since paper is formed of a hygroscopic material (e.g., pulp), it can be easily influenced by environmental humidity to absorb moisture or to dry out. As a result, the material or characteristics of the recording sheet are changed, causing dimensional error or mechanical deformation. This poses various problems in image recording.
For example, in an electrophotographic apparatus using a photosensitive sheet, the charging capacity of a recording sheet is degraded to decrease an image density or the mechanical strength of the sheet, thus making convey of the recording sheet impossible. In a plain paper copy machine, if a recording sheet absorbs moisture, a toner image cannot be satisfactorily transferred thereto, and it is difficult to reliably convey the recording sheet. In a conventional electrostatic plotter and the like, an electrostatic latent image is formed using a multistylus recording head or an ion-flow electrostatic recording head on an electrostatic recording sheet consisting of a conductive-processed paper sheet coated with a dielectric layer, and the latent image is developed by toner to form a visible image. In apparatuses of this type, when the recording sheet absorbs moisture, image density is decreased or the recording sheet is easily wrinkled. This results in poor contact between the recording sheet and the head, and makes development impossible.
As image forming apparatuses which sequentially form images of different colors on a single sheet to form a multicolor image, an electrostatic color plotter, a thermal transfer color printer, a thermal transfer color plotter, an electrophotographic color copy machine, an electrophotographic color printer, and the like are well known. In these apparatuses, if the water content of a recording sheet changes while images of different colors are being formed on a single sheet at different timings, the size of the recording sheet is changed, thus causing a shift in the colors.
In addition to such changes in recording sheets, other constituents of image recording apparatuses are also influenced by the change in environmental humidity. For example, the dry developing agent of an electrophotographic recording apparatus induces a decrease in electrical resistance or charging capacity when there is high humidity, thus causing fogging of the image. In a multistylus electrostatic recording head, electrodes may be impaired over time by rust.
As a countermeasure against the above problems, drying means using heat has been conventionally proposed. For example, Japanese Patent Disclosure No. 53-12326 discloses heating means which increases the ambient temperature in a copying machine. U.S. Pat. No. 4,259,565 also describes a similar apparatus. Japanese Patent Publication No. 56-35147 discloses a method of absorbing moisture in a copying machine using a moisture absorbent (drying agent). In addition, Japanese Patent Disclosure No. 57-78577 discloses an apparatus including a metal plate or fin for allowing condensation to take place at an airflow intake port.
However, a drying operation using heating increases the temperature in the copying machine, and can thereby cause other problems. If the relative humidity is excessively decreased by drying, there are other problems associated therewith. Moisture absorption using a drying agent requires maintenance (e.g., periodical replacement of the drying agent), and moisture absorption capacity is very small. Therefore, a satisfactory drying effect cannot be expected. In addition, even if a fin or a metal plate is arranged at an airflow intake port, condensation occurs on this portion under quite limited conditions. If air is continuously introduced into the copying machine, almost no moisture absorption effect can be expected.
FIGS. 1 to 4 explain drawbacks in a conventional recording apparatus using an electrostatic recording sheet in high humidity, and problems caused by heat-drying.
FIG. 1 is a schematic diagram of a conventional recording apparatus using an electrostatic recording sheet. Referring to FIG. 1, reference numeral 1 denotes an electrostatic recording sheet wound in the form of a roll. Recording sheet 1 is extended by feed rollers 2 and exhaust rollers 3 to be conveyed for recording. Auxiliary electrodes 4 and recording head 5 for forming an electrostatic latent image on recording sheet 1, developing head 6 for developing the latent image by toner, and fixing head 7 for fixing a developed toner image are arranged between rollers 2 and 3. The latent image is formed on sheet 1 by electrodes 4 and head 5 to which, e.g., +300 V and -300 V voltages are selectively applied by control device 8. The latent image is developed by head 6 to form a toner image, the toner image is fixed by head 7, and sheet 1 is exhausted by rollers 3.
The main drawback in the electrostatic recording apparatus with the above arrangement is that recording quality on recording sheet 1 varies widely according to the environmental humidity. This is due to a change in electrical resistance of the recording sheet caused by humidity. For example, as can be seen from the graph of FIG. 2 showing a change in surface resistance in relation to relative humidity, when the relative humidity changes from 30% to 75% at 25.degree. C., the surface resistance of the recording sheet changes from 1.5.times.10.sup.3 .OMEGA..multidot.cm to 5.times.10.sup.8 .OMEGA..multidot.cm.
Recording image quality is also degraded in a low humidity state (about 40%RH or lower). For example, since the electrical resistance of a recording sheet is normally high in this state, a sufficient charging current cannot flow and, hence, a charging potential is not increased satisfactorily, thus decreasing the recording density.
In order to eliminate this drawback, a voltage to be applied to a recording electrode is increased in the low humidity state. However, with this method, additional components (e.g., a humidity sensor) are required, resulting in an expensive, complicated apparatus. In addition, a so-called "abnormal discharging" phenomenon easily occurs. In the "abnormal discharging" phenomenon, air is ionized by the voltage applied to the recording head, and the ions cause avalanche breakdown in the recording head, thus charging non-recording portions of the recording sheet. Therefore, the "abnormal discharging" phenomenon considerably degrades recording quality.
In a high humidity state exceeding about 65%RH, a known "ghost recording" phenomenon occurs. However, even if "ghost recording" does not occur, since charges on the sheet are discharged in the high humidity state, the recording density is decreased.
FIG. 3 is a graph wherein changes in image density of images obtained by recording sheets prepared to correspond with various environmental conditions are plotted in correspondence with a change in relative humidity. Curve a indicates characteristics of a high-humidity electrostatic recording sheet; curve b, those of a low-humidity electrostatic recording sheet; and curve c, those of a multi-environmetal electrostatic recording sheet. The recording sheets indicated by curves a and b provide high recording densities under their optimal humidity conditions, but have poor stability against a change in relative humidity. Therefore, when the environmental condition changes drastically, good image density cannot be obtained by an identical recording sheet.
"Ghost recording" is a phenomenon wherein a potential around a recording head is undesirably increased, and a "shadow" image is formed on a non-recording portion as if recording had been performed. This phenomenon also considerably degrades the recording quality.
It should be noted that, e.g., in Japan, a low-humidity state (30%RH or lower) often occurs during heating in the winter, and a high-humidity state exceeding 75%RH frequently occurs during the rainy season and in the summer. Therefore, such high- and low-humidity states quite frequently occur, and hence, image quality is also frequently degraded.
The above problems can be solved by an electrostatic recording sheet whose electrical resistance does not change or cannot be easily changed even if the relative humidity changes. However, such a recording sheet has not yet been realized. For this reason, several types of recording sheets are provided and are selectively used in accordance with the environmental conditions, resulting in cumbersome operation. In addition, even though such selective use is made, it is very difficult to obtain a good image under high-humidity conditions exceeding 75%RH.
In another conventional countermeasure, an electrostatic recording sheet capable of satisfactory recording in the low-humidity state is used, and is heated and dried to be kept in the low-humidity state. For example, as shown in FIG. 4, heater 10 comprising heat radiation plate 9 is arranged adjacent to electrostatic recording sheet 1, wound in the form of a roll, to heat it. With this arrangement, when air at 25.degree. C. and 75%RH is heated to 40.degree. C., the relative humidity is decreased to about 30%. However, with this arrangement, heater 10 must always be on. For this reason, if heater 10 malfunctions, since there is the possibility of fire, quite expensive anti-malfunction equipment is required. In addition, heater 10 consumes much power (about 100 wattage or higher) and takes a great deal of time to increase the temperature to a predetermined value. This means that when heater 10 is ON/OFF controlled depending on the environmental humidity, since the humidity increases rapidly in the rainy season, an increase in temperature that heater 10 cannot cope therewith. As a result, the recording sheet undesirably absorbs moisture.
The drying operation by heating decreases the relative humidity even in the low-humidity state, and induces variations in recording characteristics due to excess drying, as described previously. In addition, when the heating/drying operation is continued until a sufficient effect can be obtained, it tends to increase the overall temperature of the apparatus. This often adversely influences electronic circuits and other components of the apparatus as well as such things as the developing agent liquid. As shown in FIG. 4, when recording sheet 1 in the form of a roll is used, the paper is wound to maintain a given tension. If the humidity changes, recording sheet 1 is extended or contracted to cause distortion, and this distortion appears as wrinkles which interfere with uniform charging and development.
In an electrostatic image forming apparatus, high humidity adversely influences not only the recording sheet but also a recording head. For example, in a multistylus electrostatic recording head, wires constituting the multistylus rust in the high-humidity state, thus degrading recording characteristics. In an ion-flow recording head (to be described later), recording characteristics are also degraded by poor insulation due to moisture absorption and low resistance of foreign materials which become attached near the head.
Problems caused by moisture absorption and drying of a recording sheet is significant in a color image forming apparatus. For example, images of different colors are formed during respective reciprocating cycles while the recording sheet is reciprocated with respect to the recording head, so that the images are time-serially overlaid on each other. This arrangement is well known as per Japanese Patent Disclosure Nos. 50-80713, 57-114158, and the like. However, in the recording apparatus of this type, if the recording sheet absorbs moisture or is dried during recording, the size thereof changes, causing shifting of the colors. In order to prevent this, as disclosed in Japanese Patent Disclosure Nos. 57-124753 and 57-122455, marks are recorded on a non-recording portion of the recording sheet at equal intervals, and a change in size is calculated by reading a distance between the marks to control recording dot positions accordingly. However, such processing is complicated, and even if it is executed, a change in the widthwise size of the recording sheet cannot be corrected.
As described above, problems in the image recording apparatuses frequently occur when the recording sheet or the apparatus is in the high-humidity state, and conventional countermeasures for these problems provide insufficient effects or induce additional problems.